1. Field of the Invention
The present invention relates to headphone apparatus, and more particularly, to a headphone apparatus having improved frequency characteristics in low frequency range. This invention is applicable, for example, to an inner-type headphone apparatus.
2. Description of the Background Art
The structure of a conventional inner-ear type headphone apparatus will be explained hereinafter. FIG. 1 is a sectional view of a structure of a conventional headphone apparatus.
Referring to FIG. 1, a dome-like diaphragm (vibration plate) 31 and a driving portion 32 are formed at the opening end side of a cylinder portion 34 of a case 33. Driving portion 32 drives diaphragm 31. Cylinder portion 34 has a bottom at the lower portion thereof. Driving portion 32 is constituted by a magnetic circuit portion comprising a magnet 36. The magnetic field of magnet 36 acts on a voice coil 35. Voice coil 35 is joined to diaphragm 31. A flow of audio signal current to voice coil 35 causes the vibration of diaphragm 31. This vibration generates compression waves in the space at the forward side of diaphragm 31 (the upper side in the figure) to produce sounds.
There is a back cavity 38 at the rear side of driving portion 32. This back cavity 38 communicates with diaphragm backside space 37 between diaphragm 31 and driving portion 32. Back cavity 38 suppresses the influence of compression waves generated also in the air at the rear side of diaphragm 31 to enhance the low frequency range.
A backside wall 39 of case 33 covering back cavity 38 from the backside is formed with a plurality of slit-like through-holes 40 for adjusting frequency characteristics. These through-holes 40 are provided to suppress reduction in sound output of middle frequency range specific to the configuration of back cavity 38. In other words, these through-holes 40 have opening configuration in accordance with the middle frequency range thereof. If through-holes 40 are not provided, a particular frequency range reflected from backside wall 39, i.e. the sound wave of middle frequency range which is of opposite phase interferes with the sound wave emitted forwards from diaphragm whereby the output of middle frequency range is reduced, for example. If through-holes 40 are provided as slits in accordance with the middle frequency range, at least one portion of the sound wave of the particular middle frequency range escapes backwards through-holes 40. This will reduce the interference between sound wave from diaphragm 31 to the forward direction and the sound wave of the particular middle frequency range. This results in improvement of frequency characteristics that are output from diaphragm 31.
In the above mentioned conventional headphone apparatus, the sound wave of middle frequency range is attenuated to some extent upon passing through-holes 40 having narrow gaps. Therefore, the intensity of sound of middle frequency range leaking to external space (outside the headphone apparatus) is lowered. However, sound wave of high frequency range is hardly attenuated even when passing through-hole 40 provided in accordance with the middle frequency range. Sound wave of high frequency range leak to external space through these through-holes 40. The sounds of high frequency range are enhanced and that will annoy people close to the person wearing the headphone. Thus, there was a problem that the sound of high frequency range leaking from a headphone annoys people close to a person wearing a conventional headphone.
Japanese Patent Publication No. 64-8519 discloses a conventional inner-ear headphone apparatus. In Japanese Utility Model Laying-Open No. 61-195188, a structure of a headphone is disclosed capable of changing the capacity of a cavity corresponding to back cavity 38 shown in the aforementioned FIG. 1. The above publications do not have any recitation of a headphone structure for reducing the leakage of high frequency range sound.